media: Documentation: Use right function to test device power state

media: Documentation: Use right function to test device power state

Tell driver authors to use pm_runtime_get_if_active() instead of
pm_runtime_get_if_in_use() to check the device's power state in the s_ctrl
callback. pm_runtime_get_if_active() is the right function to use here
since it returns non-zero if the device is powered on rather than its
PM runtime usage_count is non-zero.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>